Method of producing and treating aluminum alloy castings



Patented Nov. 25, 1941 METHOD OF PRODUCING AND TREATING ALUMINUM ALLOYCASTINGS Walter Bonsack, South Euclid, and Walter M. Weil, ShakerHeights, Ohio, assignors to The National Smelting Company, Cleveland,Ohio, a corporation of Ohio No Drawing. Application December 6, 1939,Serial No. 307,882

Claims.

This invention relates to aluminum base alloys, and more particularly toa method of heat treating aluminum base alloy castings containing one ormore hardening ingredients.

It is an object of this invention to provide a method for increasing thestrength and hardness of heat treatable aluminum alloy castings byheating the castings for a relatively. short time.

It is another object of this invention to provide a simpler and moreefiicient method of heat treating aluminum alloy castings in acomparatively short time to give them considerably improved properties.

In order to improve the strength and hardness of aluminum alloy castingsby heat treatment, a solid solution having a substantial amount ofhardening constituents dissolved therein should be first obtained, and asubstantial proportion of these dissolved hardening ingredients shouldbe precipitated in highly dispersed form from the solid solution. In anygiven heat treatable alloy, the larger the amount of hardeningingredients in the solid solution the greater is the potentiality forhardening.

The solid solution may be prepared by the socalled solution heattreatment of the metal as described in the Bureau of Standards BulletinNo. 337, of 1919, and later in the Jefiries and Archer Patent No.1,572,487, granted February 9, 1926. By this method the castings areheated at temperatures around 900 F. to 1000 F. for at least sevenhours, and usually for a considerably longer time, to dissolve thehardening constituents.

In carrying out such solution heat treatments the time of holding thecasting at the high temperature is important. A relatively long heatingperiod is generally necessary to cause a suflicient proportion of theprecipitated hardening ingredients to be dissolved to accomplish a sub-.

stantial improvement in properties. Even after the solid solution hasbeen formed in the manner above set forth, the hardening constituentswill be precipitated again in sulzstantially ineffective form if thecasting be allowed to pass slowly through the temperature range whereprecipitation may occur relatively rapidly. It is therefore desirable,in order toobtain th benefits of solution heat treatment, that the hotcasting be quenched relatively quickly to the temperature of employed,so that the potentiality for hardening and strengthening by utilizingone or the wellknown aging processes is considerably less. This methodhas not, therefore, displaced the solution treatment in the preparationof those castings where the most improved properties, such as strength,ductility and hardness, are desired.

When a casting is removed from the mold relatively quickly after it issolidified and quenched, not all of the hardening constituents areprecipitated from the solid solution. It has been found that such acasting, however, even though quenched from the mold may be heated fromroom temperature througha temperature range where precipitation of thehardening constituents usually occurs, up to the ordinary solution heattreating temperatures, without precipitating a substantial amount ofdissolved hardening ingredients. If this is done, one may heat thequenched casting at a solution heat treatment temperature, which may beconsidered asa temperature where solubilization of hardeningingredientsof the alloys occurs, for a comparatively short time to produce resultsequal or superior to those obtained by the longer solution heattreatment.

In accordance with our invention, heat treatable aluminum alloy castingsof superiorstreng'th and hardness are obtained by quenching the castingsafter solidification in the mold, and while they are hot enough to havea substantial quantity of the hardening ingredients in solid solution,heating the quenched castings atsolution heat treatment temperatures fora comparatively short time, again quenching the castings and giving thema suitable aging treatment.

This invention applies to articles cast from aluminum alloys containinghardening constituents, whether such articles be cast in sand or metalmolds. The articles are-preferably removed'i'rom the mold and quenchedas soon'as is practical after solidification oi the'metal takes.place,'so as to retain larger amounts of hardenthe precipitation zone,or below, to avoid appreciable crystallization of the hardeningconstituents during the cooling.

A solid solution of hardening ingredients may ing constituents in solidsolution. Even when the castings have cooled below the ordinary solutionheat treating temperatures before they are quenched, sufllcient of theingredients are 'retained in solid solution to substantially shorten thetime of heating subsequently required. It is preferred, however, thatthe castings be removed from the molds and quenched at a temperatureabove 850 F. or 900 F. depending on the composition of the alloy.

The quenched castings which have a substantial'qauntity of dissolvedhardening constituents should preferably be heated as rapidly aspossible through the precipitation temperature range to the solutionheat treating temperature. They may be heated in any suitable heattreating furnace. When heated in a gaseousgatmospherathe furnace shouldpreferably be of such a design as to cause the heating of the casting tobe relatively rapid.

The solution temperatures vary of course with the alloy used, but maygenerally be considered as temperatures which are slightly below themelting point of the eutectic, and for most aluminum alloys are usuallyin the range of 850 F. to 1050 F. The preferred solution temperature forany heat treatable aluminum alloy is to 20 below the melting point ofthe eutectic.

For example, the aluminum-silicon alloys are treated at temperaturesbetween 950 F. and 1050 F., whereas the aluminum-copper alloys aretreated at lower temperatures, between 900 F. and 1000 F., andaluminum-zinc alloys are heated to temperatures still lower, such as 850F. to 950 F. When the previously quenched casting is heated a relativelyshort time in the temperature range appropriate to the alloy, a solutionof the hardening ingredients is obtained which is equal to or superiorto that obtained by -the relatively long conventional heat treatmentsheretofore used.

In practicing the invention, an hour or less is usually suiiicient at asolubilizing temperature, and only seldom is it desirable to heat thealloy at such temperature for as long as about 2 or 3 hours. It will,therefore, be seen that a tremendous saving in cost of heat treating ismade by the application of this invention. Thus, it has been found thatcastings quenched from the mold and heated for only about an hour at asuitable temperature for the alloy and then again quenched, have evengreater potentiality for hardening and strengthening when suitably agedthan the usual castings which have been heated 8 or 10 hours at 950" F.and quenched.

Although with castings previously quenched from the mold a heatingperiod of minutes or s so at the solution or solubilizing temperatureproduces marked improvements, it is preferred that the heating becontinued for a longer time, such as about an hour or two, to obtain amore uniform product having a larger percentage of hardeningconstituents in solid solution. The time to' produce a desired solidsolution of hardening ingredients is of course dependent on thetemperature, and the higher the temperature or with a given alloy thenearer the temperature used is to the melting point of the eutectic ofthe alloy, the shorter will be the time required. It has been found thatcastings having excellent properties may be obtained by holding them ata suitable solubilizing temperature for about a half hour or so,quenching and aging.

The quenching may be in any medium which cools the casting suflicientlyrapidly to retain the hardening constituents in solid solution. However,as a general rule, when cooling or quenching of the casting from thesolution heat treating temperature is relatively rapid, a greaterpotential improvement of the hardness and strength of the metal of thecasting may be obtained. The casting may be quenched to roomtemperature, or to the temperature at which it is desired to age thecasting, thus avoiding the reheating of it to the aging temperature. Inorder to develop the inherent physical properties of the casting, asuitable aging treatment is usually given castings after quenching.

If the alloy used is one which readily increases in hardness at roomtemperature, the treated casting may be aged at room temperature. It is,however, often desirable or necessary that the aging be carried on atelevated temperatures, such as that of boiling water or temperatures upto about 450 F., to improve the characteristics of the castings.

The aging time and aging temperatures erally used for solution heatedalloy castings are applicable to the castings heat treated in accordancewiththis invention. The aging time depends upon the particular alloybeing aged and the properties of the metal desired. For any givenproperties, the required aging time will depend upon the temperature,and with a higher temperature a shorter aging time may usually be used.It is rarely necessary to age more than fifteen hours, and for manycastings the time may be considerably less.

The method set forth above is applicable to heat treatable aluminumalloys, such as those having one or more of theusual commercial alloyingconstituents, as copper, zinc, magnesium, silicon, manganese and nickel.

The following example illustrates the invention:

A molten aluminum alloy containing 4% copper, 2% silicon, 1% iron and0.5% zinc was poured into a permanent mold at operating temperature. Thetest bars were quenched in water from the mold at a temperature betweensolidification temperature and mold temperature. The quenched castingswere transferred to a heat treating furnace, heated for 1 hours or so atabout 950 F., again'quenched in hot water and aged 2 hours at 212 F.When tested, the hardness of the casting was 65 Brinell, the tensilestrength 40,000 lbs./sq. in., and elongation 7.5%.

When the same alloy was cast in the same manner and in the same mold butslowly cooled to room temperature, it had to be heated for about '7hours at about 950 F. to get about 37,000 lbs./sq. in. tensile strengtha hardness of 60 Brinell, and elongation of 7%.

Furthermore, it is to be understood. that the particular compoundsdisclosed and the procedure set forth are presented for purposes ofexplanation and illustration, and that various equivalents can be usedand modifications of said procedure can be made without departing fromour invention as defined in the appended claims.

What we claim is: 1. The process of producing an aluminum alloy casting,which comprises pouring the molten metal in a gravity permanent mold,re-

ening ingredients in solid solution, heating the quenched casting to atemperature at which solubilization of the hardening ingredients occurs,holding the casting at such temperature for fifteen minutes to threehours, and again quenching the casting from a solubilizing temperature.

2. In a process for producing -castings from aluminum alloys containingcopper, the steps which comprise quenching the casting from. the moldrelatively quickly after it has solidified and before it has cooledbelow about 850 F., so

that an efiective proportion of the hardening ingredients is still insolid solution, heating the quenched casting to a temperature to causesolubilization of hardening ingredients not in solution, maintaining thecasting at such solubilizing temperature for less than three hours,again quenching the casting from a solubilizing temperature' to asuitable aging temperature, and aging th casting.

3. The process of producing an aluminum alloy casting, which comprisespouring the molten metal into a gravity mold, removing the casting fromthe mold promptly after solidification, quenching before it has cooledbelow about 850 F., heating the quenched casting between about 850 F.and the melting point of the lowest melt-' ing eutectic in the alloy,maintaining this temperature for from fifteen minutes to one hour, againquenching the casting from a temperature above about 850. F., andartificially aging the casting above room temperature and below about450 F. to improve its properties.

4. The process of producing an aluminum alloy casting, which comprisespouring the molten metal in a mold, removing the casting from the moldbefore it has cooled to 850 F. and quenchingit, heating the quenchedcasting 'at temperatures between 850 F. and 1050 F. for fifteen minutesto three hours to cause solubllization of the hardening ingredients, andagain quenching the casting. I

5. The process of producing an aluminum alloy casting, which comprisespouring the molten metal into a gravity mold, quenching the casting fromthe mold before it has cooled below about 850 F., so that a substantialquantity of hardening ingredients is in solid solution, heating thequenched casting at a temperature at which ,solubilization of thehardening constituents of the alloy occurs, and quenching the castingfroma solubilizing temperature.

WALTER BONSACK. WALTER M. WEIL.

